The invention relates to an optical modulation device for coupling an entering radiation field to at least one of two exiting radiation fields, comprising an acousto-optical modulator, a first sound (e.g., acoustic) wave field travelling through the acousto-optically active medium of this modulator in a sound propagation direction and by means of a first acousto-optical modulation dividing a radiation field incident in an entry direction and coupled to the entering radiation field essentially into a transmitted branch propagating in the direction of a beam axis of the incident radiation field and a diffracted branch extending with its beam axis at an angle of diffraction of the first order in relation to the beam axis of the transmitted branch, wherein an angle bisector between the beam axis of the incident radiation field and the beam axis of the diffracted branch extends approximately parallel to the sound propagation direction of the sound wave field.
Optical modulation devices of this type are known, for example, from the book xe2x80x9cFundamentals of Photonicsxe2x80x9d of Bahaa E. A. Saleh and Malvin Carl Teich, John Reiley and Sons, New York, 1991, page 799 to page 831.
In the case of these modulation devices there is, however, the problem that when these modulation devices are intended to be used for the switching of radiation fields, diffraction efficiencies of up to 100% must be achieved and these can be achieved, if at all, only with considerable resources.
The object underlying the invention is therefore to improve an optical modulation device of the generic type in such a manner that as efficient a switching of the incident radiation field as possible between the exiting radiation fields is possible.
This object is accomplished in accordance with the invention, in an optical modulation device of the type described at the outset, in that a radiation guide system is provided which deflects the diffracted branch resulting during the first acousto-optical modulation and the transmitted branch such that with their beam axes extending approximately at the angle of diffraction of the first order relative to one another they interact with a travelling second sound wave field having approximately the same frequency as the first sound wave field in order to generate a second acousto-optical modulation, whereby essentially a diffracted and a transmitted branch respectively result from the deflected, diffracted branch and the deflected, transmitted branch, that the direction of propagation of the second sound wave field is aligned relative to the deflected, diffracted branch and the deflected, transmitted branch such that the transmitted branch resulting from the deflected, diffracted branch and the diffracted branch resulting from the deflected, transmitted branch propagate in approximately the same direction, are superimposed at least partially and thereby have essentially the same frequency so that these at least partially superimposed branches form a first radiation field as a result of essentially constructive interference, and in addition the transmitted branch resulting from the deflected, transmitted branch and the diffracted branch resulting from the deflected, diffracted branch propagate in the same direction, are at least partially superimposed and thereby have essentially the same frequency so that these at least partially superimposed branches form a second radiation field as a result of essentially destructive interference, and that the first radiation field is coupled to the first exiting radiation field and the second radiation field to the second exiting radiation field.
The advantage of the inventive solution is to be seen in the fact that as a result of the inventive execution of the second acousto-optical modulation in such a manner that two respective branches result which are superimposed and have the same frequency, constructive and destructive interference can respectively be used to form the first radiation field and the second radiation field from the respective branches.
As a result, large variations in intensity between the first and second radiation fields are possible at a low diffraction efficiency. For example, it is sufficient to be able to operate the first acousto-optical modulation and the second acousto-optical modulation with a diffraction efficiency of at the most 50% in order to couple the entering radiation field completely into the first radiation field or the second radiation field.
This allows, in particular, use of simple optical modulators and a lower high-frequency power for generating the sound wave fields and so, as a result, the acousto-optical modulators can, altogether, be constructed and operated more simply.
Particularly high intensities of the first radiation field may be obtained when the branches forming the first radiation field are superimposed in essential parts.
A partial superposition is also sufficient with respect to the branches forming the second radiation field, wherein for achieving intensities which are as high as possible the branches forming the second radiation field are likewise superimposed in essential parts where possible.
With respect to generating the first and second sound wave fields it would, in principle, be conceivable to use different sound generators with different sound frequency generators.
However, in order to ensure that the frequencies of the first and second sound wave fields are as close to one another as possible or even identical it is preferably provided for the first and second sound wave fields to be generated with a single sound frequency generator.
Furthermore, in order to achieve as uniform a diffraction efficiency as possible during the first acousto-optical modulation and the second acousto-optical modulation it is preferably provided for the first and the second sound wave fields to have amplitudes of essentially the same size.
In the case of an inventive modulation device a concept which is as simple as possible provides for the first acousto-optical modulation and the second acousto-optical modulation to take place in separate acousto-optical modulators so that it is also possible, due to this separation of the acousto-optical modulators, to vary the individual, acousto-optical modulations with respect to the diffraction efficiency.
This solution is particularly favorable when the diffraction efficiency of the first acousto-optical modulation or the second acousto-optical modulation is intended to be different in relation to the diffraction efficiency of the respectively other acousto-optical modulation.
With this solution, it is possible, in particular, to select optional intensities of the first radiation field and the second radiation field.
However, in order to be able to ensure in as simple a manner as possible that the first and the second acousto-optical modulations take place with the same frequency and under the same overall conditions, it is preferably provided for the first acousto-optical modulation and the second acousto-optical modulation to take place in the same acousto-optical modulator, in which a single sound wave then propagates and a single grating of wave fronts is generated, at which the two acousto-optical modulations take place.
Even when carrying out the two acousto-optical modulations in one and the same modulator it is possible to have the first acousto-optical modulation and the second acousto-optical modulation carried out in the same acousto-optical modulator essentially spatially separable so that a simple separation of the first radiation field and the second radiation field from the incident radiation field is also possible.
The construction of the radiation guide device and the radiation guidance itself may, in particular, be simplified even more when the first acousto-optical modulation and the second acousto-optical modulation take place essentially in the same volume area of the acousto-optical modulator so that, as a result, it is also ensured that the same conditions exist for the two acousto-optical modulations.
No further details have so far been given with respect to the alignment of the beam axes during the two acousto-optical modulations relative to the respective sound propagation direction. One advantageous embodiment provides for a beam axis of the incident radiation field and a beam axis of the diffracted branch resulting during the first acousto-optical modulation as well as a beam axis of the transmitted branch to define a first plane of modulation approximately parallel to the first sound propagation direction and during the second acousto-optical modulation for the beam axes of the diffracted and transmitted branches resulting from the diffracted branch and the transmitted branch to define a second plane of modulation approximately parallel to the second sound propagation direction.
As a result of the fact that the first acousto-optical modulation and the second acousto-optical modulation take place in a respective plane the two acousto-optical modulations may be separated or combined as required by way of suitable positioning of the planes.
One advantageous embodiment, for example, provides for the first and the second planes of modulation to be located in a common plane, whereby the beam guidance is simplified during the generation of the deflected, diffracted branch and the deflected, transmitted branch, wherein it is not automatically determined as a result that the two acousto-optical modulations cannot take place separately in the acousto-optical modulator.
Furthermore, it is also not determined as a result that one acousto-optical modulator must automatically be used. Even when the first and second planes of modulation are located in a common plane, it is still possible to use two acousto-optical modulators which are separate from one another.
A further, advantageous solution provides for the first and second planes of modulation to be arranged to as to be offset parallel to one another. This arrangement of the two planes of modulation creates the possibility of separating the first radiation field and the second radiation field from the incident radiation field in a simple manner even when both modulations take place in a single acousto-optical modulator.
Another favorable possibility for the separation between the incident radiation field, on the one hand, and the first radiation field and the second radiation field which result following the second acousto-optical modulation, on the other hand, consists in having the first plane of modulation and the second plane of modulation extending at an angle to one another.
This solution also does not automatically require the first acousto-optical modulation and the second acousto-optical modulation to be carried out in a single acousto-optical modulator. With this solution, as well, the two acousto-optical modulators may be arranged separately from one another.
When both acousto-optical modulations take place in one acousto-optical modulator it is advantageously provided for the first plane of modulation and the second plane of modulation to intersect and have a line of intersection extending parallel to the sound propagation direction of the sound wave field.
A particularly favorable solution provides for the line of intersection to extend through the volume area of the acousto-optical modulator, in which the first acousto-optical modulation and the second acousto-optical modulation take place so that the same grating of wave fronts is, where possible, essentially responsible for the two acousto-optical modulations.
With respect to the design of the radiation guide system no further details have been given in conjunction with the preceding explanations concerning the individual embodiments. One advantageous embodiment, for example, provides for the radiation guide system to divert the diffracted branch resulting during the first acousto-optical modulation and the transmitted branch from the first plane of modulation into the second plane of modulation and then in the second plane of modulation to supply them to the second acousto-optical modulation as a deflected, diffracted branch and as a deflected, transmitted branch.
In this respect, when two acousto-optical modulators are used the radiation guide system can be designed such that it deflects the diffracted branch and the transmitted branch from the first acousto-optical modulator and supplies them to the second acousto-optical modulator.
In the case of a single acousto-optical modulator the radiation guide system is preferably designed such that it returns the diffracted branch resulting during the first acousto-optical modulation and the transmitted branch to the same acousto-optical modulator as a deflected, diffracted branch and a deflected, transmitted branch.
Particularly favorable conditions are present for the second acousto-optical modulation when the diffracted branch runs to the second acousto-optical modulation approximately parallel to the transmitted branch resulting during the first acousto-optical modulation.
Furthermore, it is preferably provided for the transmitted branch to run to the second acousto-optical modulation approximately parallel to the diffracted branch resulting during the first acousto-optical modulation.
These conditions with respect to the parallel course of the various branches may, however, be fulfilled only when the first and second planes of modulation are either offset in parallel or coincide in one plane.
With respect to the radiation guide properties of the radiation guide system, no further details have been given in conjunction with the preceding explanations concerning the individual embodiments.
One particularly advantageous embodiment of the inventive radiation guide system, for example, provides for this to deflect the diffracted branch and the transmitted branch such that the optical path between the first acousto-optical modulation and the second acousto-optical modulation is approximately the same in both branches.
The optical path of the radiation guide system can expediently be selected such that the relative phase position of the individual branches of the branches interfering with one another can be determined in a defined manner for the formation of the first radiation field and the second radiation field.
An inventive radiation guide system can be realized in the most varied of ways.
One possibility can be brought about by way of light guides, wherein a respective, separate light guide can be provided, for example, for the diffracted branch 40B and the transmitted branch 40T.
The return of the individual branches is also possible, for example, due to the fact that the transmitted branch is coupled into one end of a light guide and the diffracted branch into the other end and the returned branches then exit again at the respectively opposite ends.
Another solution provides for the radiation guide system to have at least two beam deflections which cause the branches running apart at the angle of diffraction to run towards one another again at the angle of diffraction, for example, as returned branches.
The beam deflections are preferably formed by reflector surfaces.
In this respect, it is preferably provided for the two reflector surfaces to extend towards one another at an angle of less than 90xc2x0.
Another solution provides for a reflector and an optical imaging means, for example, an optical telescope comprising at least two lenses.
No further details have so far been given with respect to the design of the radiation return system.
One advantageous embodiment provides for the radiation return system to align the returned branches such that they run towards one another in the acousto-optical modulator such that the transmitted and diffracted branches resulting from them again result as close as possible to one another.
This is preferably brought about such that the branches intersect one another again in the acousto-optical modulator at least in sections, even better essentially completely.
In one case, it is provided for the respective returned branch to run parallel to the respectively other branch in the acousto-optical modulator.
A particularly favorable solution, in particular, with a view to the formation of a returning first radiation field which is as uniform as possible from a spatial point of view and a second radiation field which is as uniform as possible from a spatial point of view provides for the first radiation return system to cause the returned branches to extend in the optically active volume of the acousto-optical modulator such that they intersect one another at least partially approximately in the junction area of a division into the transmitted and the diffracted branches, wherein the one respective returned branch preferably extends approximately congruent but with an opposite direction of propagation to the respectively other branch in the optically active volume area within the acousto-optical modulator. As a result, the transmitted and diffracted branches resulting again from the returned branches also coincide essentially with one another.
Another alternative solution provides for the radiation guide system to return the returned branches as branches running apart from one another in the acousto-optical modulator, i.e. the respective returned branches, formed, for example, due to reflection not only of the transmitted branch but also of the diffracted branch, extend in the acousto-optically active medium within the acousto-optical modulator as branches running apart from one another.
It is also conceivable with this relative orientation of the returned branches to couple the transmitted branch and the diffracted branch into a respective light guide and to cause them to exit from this light guide again at the other end with a corresponding alignment.
A particularly simple radiation guide system is preferably constructed such that it has a single reflector.
The one reflector is preferably aligned such that a radiation field impinging on it is reflected back at an angle of return reflection which corresponds to the angle of diffraction of the first order of the acousto-optical modulator.
In this respect, the reflector is preferably designed such that it has a flat reflector surface which is aligned in accordance with the angle of return reflection.
The reflector surface may be part of a reflector separate from the acousto-optical modulator.
Another advantageous solution provides for the acousto-optical modulator to bear the reflector on a side surface on the exit side for the radiation field incident in it.
In the simplest case, such a reflector may be produced on the side surface of the acousto-optical modulator on the exit side when the side surface of the acousto-optical modulator is covered, preferably coated, with a reflector layer.
With all the variations of the inventive solution, with which the returned branches likewise enter the optically active volume area of the acousto-optical modulator as branches running apart from one another, the branches forming not only the returning radiation field but also the radiation field coupled out are offset in a direction transverse to their direction of propagation.
This offsetting does, however, preferably lie within the cross section of the radiation field forming altogether and so this does not have any appreciable affect when the reflector layer is arranged close to a junction of the branches in the acousto-optical modulator, i.e. the acousto-optical modulator has an extension in the direction of propagation of the incident radiation field which is as limited as possible and is required only for a sufficient interaction.
One advantageous possibility for separating entering radiation field and exiting radiation fields despite coinciding planes of modulation provides for a separation of the entering radiation field from at least one of the exiting radiation fields to be achievable in that the first radiation field extends at a distance from the incident radiation field and thus is separate from it. As a result, the first radiation field does not run back approximately in the direction of the incident radiation field or overlap with it but the first radiation field is rather completely separate from the incident radiation field.
In this respect, it is also even more advantageous when the second radiation field extends at a distance from the branch diffracted away from the incident radiation field.
With this solution, the separation of entering radiation field and exiting radiation field is already ensured by the separation of the first and the second radiation fields from the incident radiation field since no optical components whatsoever are required in order to bring about a separation of this type.
This may be achieved particularly simply in one embodiment of the inventive solution in that the returned transmitted branch and the returned diffracted branch interact in an area of the acousto-optical modulator which is arranged so as to be offset in relation to the area of the division of the incident radiation field into the diffracted and transmitted branches in a direction approximately parallel to the direction of propagation of the sound wave, i.e. an offsetting is possible not only in the direction of the direction of propagation of the sound wave but also in the opposite direction to the direction of propagation of the sound wave and as a result of this parallel offsetting the returned transmitted and the returned diffracted branches can again interact with one another and the first radiation field and the second radiation field then result from this interaction and these fields extend spatially separate from and, in particular, not overlapped by the incident radiation field so that the first radiation field and the second radiation field can directly form the first exiting radiation field and the second exiting radiation field, respectively.
One particularly advantageous embodiment provides for the radiation return system and the acousto-optical modulator to interact such that a first radiation field and a second radiation field exit from the acousto-optical modulator and these fields propagate in directions which have at least one directional component extending in the opposite direction to the direction of propagation of the incident radiation field.
A particularly favorable arrangement of acousto-optical modulator and radiation return system provides for the first radiation field exiting from the acousto-optical modulator on a side located opposite the radiation return system to extend approximately parallel to the incident radiation field and for the second radiation field to extend at an angle of diffraction of the first order in relation to the first radiation field.
With such an arrangement, particularly favorable ratios may be achieved by way of two-time diffraction effects which are respectively based on the same principle and, together with the radiation return system, cause altogether superposition effects to be generated which correspond to those of an xe2x80x9canti-resonant ring interferometerxe2x80x9d.
In order, for example, in the preceding arrangement to separate the exiting radiation field to be formed from the first radiation field from the entering radiation field it is preferably provided for at least one incident radiation field polarized in one direction to be generatable in the modulator unit from the entering radiation field.
In order, for example, in the case of the polarization explained above to avoid losses in intensity in the case of unpolarized light it is preferably provided for two incident radiation fields with directions of polarization at right angles to one another to be generatable from the entering radiation field.
A particularly favorable separation of entering radiation field and at least one of the exiting radiation fields is possible due to the fact that the polarized first radiation fields resulting from the entering radiation field experience a rotation of polarization through altogether xc2x190xc2x0 until the exiting radiation fields are formed.
Such a rotation of polarization can be achieved, for example, in that the respective incident radiation field experiences a rotation of polarization through 45xc2x0 in a polarization-influencing element and the first radiation field exiting from the acousto-optical modulator experiences a further rotation through 45xc2x0 as a result of the same polarization-influencing element.
Alternatively thereto it is, however, also conceivable to use polarization-influencing elements which turn the direction of polarization of the incident radiation field through 90xc2x0 and leave unaffected the direction of polarization of the first radiation field passing through them.
One embodiment of a modulator unit which separates the entering radiation field from at least one of the exiting radiation fields provides for an optical diode, on which the entering radiation field impinges and from which the returning first radiation field also exits in the form of one of the exiting radiation fields.
A particularly advantageous inventive embodiment provides for a radiation return system to return the branches, which propagate in the acousto-optical modulator in their exiting directions and are incident in the radiation return system, to the acousto-optical modulator at a respective angle to the exiting directions which corresponds approximately to the angle of diffraction of the first order, for the radiation return system to be arranged such that it returns the transmitted branch propagating in the acousto-optical modulator in exiting direction and the corresponding diffracted branch propagating in exiting direction to the acousto-optical modulator such that the returned transmitted branch and the returned diffracted branch extend in the acousto-optical modulator approximately parallel to the diffracted branch propagating in exiting direction or approximately parallel to the transmitted branch propagating in exiting direction, and for the transmitted and diffracted branches respectively resulting from the returned transmitted branch and the returned diffracted branch to be superimposed to form a first radiation field and a second radiation field, each of which is coupled to one of the exiting radiation fields.
The advantage of this embodiment is to be seen in the fact that as a result of the inventive design of the radiation return system each returned branch extends in the acousto-optical modulator approximately parallel to the respectively other branch and is divided in the modulator into a transmitted branch and a diffracted branch so that a first or returning radiation field exits from the acousto-optical modulator which is approximately parallel to the incident radiation field but propagates in the opposite direction and has the branches of the radiation field which have resulted on their way from the incident radiation field to the first radiation field due to a one-time diffraction in the acousto-optical modulator whereas a second radiation field exiting from the acousto-optical modulator propagates approximately parallel to the diffracted branch and thus at the angle of diffraction of the first order in relation to the first radiation field and has the branches which have resulted on their way from the incident radiation field to the second radiation field either as a result of no diffraction whatsoever or as a result of a two-time diffraction, wherein the first exiting radiation field is formed from the first radiation field and the second exiting radiation field from the second radiation field.
In the case of the inventive solution, the acousto-optical modulator can, for example, be operated together with the radiation return system as a type of anti-resonant ring interferometer, wherein a complete coupling of the incident radiation field into the first, returning radiation field is already possible at a diffraction efficiency of the acousto-optical modulator of approximately 50% since the branches which have been diffracted one time are constructively superimposed whereas the transmitted branch and the branch diffracted two times, which are superimposed destructively to form the second radiation field, can cancel one another out, whereby an acousto-optical modulator which is of a simple construction and operated with simple means can already be used.
The operation of the acousto-optical modulator with the radiation return system as xe2x80x9ca type of anti-resonant ring interferometerxe2x80x9d is to be understood such that the known xe2x80x9canti-resonant ring interferometerxe2x80x9d represents the starting point for considerations but it has also to be taken into account that the acoustic grating moves along in time and thus an acoustic grating altered as a result of the transit time results for the returned branches. Furthermore, the shift in frequency in the diffracted branch and the length of the path of the branches returned again to the acousto-optical modulator by the radiation return system have also to be taken into consideration.
In addition, when the acousto-optical modulator is not acted upon with a sound wave and thus has the diffraction efficiency zero the modulator allows an essentially complete coupling of the incident radiation field into the second divided radiation field on account of the transmitted branches exclusively being formed.
The inventive solution thus creates the possibility, despite one or two acousto-optical modulators which are of a simple construction and merely have to achieve values of the diffraction efficiency in the range of approximately 0% to approximately 50%, of changing efficiently between a maximum coupling of the entering radiation field to the first exiting radiation field or to the second exiting radiation field.
In principle, it would be conceivable to operate the acousto-optical modulator or modulators only with two different diffraction efficiencies in order to achieve the desired modulation effects, i.e. a switching over from the first exiting radiation field to the second exiting radiation field.
It would, for example, be conceivable to operate the first and second acousto-optical modulators at a diffraction efficiency of either approximately 0% or approximately 50%.
It is, however, also conceivable within the scope of the inventive solution for the acousto-optical modulator to be operable in the range between a diffraction efficiency of approximately 0% and approximately 50% so that all the possible different degrees of coupling between the entering radiation field and the two exiting radiation fields can be set.
The inventive modulation device can, in principle, be used as required when it is a question of modulating radiation fields. For example, the inventive modulation device may be used as an external element, with which an external modulation of radiation fields and/or a mixing of radiation fields and/or also a shift in frequency of radiation fields can be carried out.
The inventive modulation device may be used advantageously, in particular, when this is arranged in an amplifying radiation field of a laser amplifier so that the advantages of the acousto-optical modulation can be used for coupling radiation fields in and out.
A particularly favorable solution provides for the modulation device to have an amplifying radiation field of a feedback laser amplifying system passing through it.
In this respect, it is possible to integrate the inventive modulation device into the laser amplifying system as a separate component.
The inventive modulation device may be used particularly favorably when this is part of a feedback laser amplifying system, i.e. not only modulates the amplifying radiation field but also serves directly for the feedback thereof.
This may be realized particularly simply when the radiation guide system of the inventive modulation device is an amplifying radiation return system of the feedback laser amplifying system.
In addition, the invention also relates, however, to a laser amplifying system comprising a feedback optical amplifier with two amplifying radiation return systems, an optical volume area which extends between the amplifying radiation return systems and passes through a laser-active medium and within which an amplifying radiation field is formed, i.e. results or is amplified, an acousto-optical modulator which has the optically active volume area and the radiation field passing through it and from which acoustic wave fronts propagate in a sound propagation direction and generate a grating, by means of which an incident amplifying radiation field can be divided into a respective transmitted branch and a respective diffracted branch extending at an angle of diffraction of the first order in relation to the transmitted branch.
Laser amplifying systems of this type are known, for example, from the book xe2x80x9cSolid-State Laser Engineeringxe2x80x9d by Walter Koechner, Springer Series in Optical Sciences, ISBN 3-540-60237-2, 1996, pages 494 to 499.
With such a laser amplifying system, the acousto-optical modulator is used such that the transmitted branch is coupled back in the resonator and the resonator losses can be modulated due to division of the incident radiation field into the transmitted branch and the diffracted branch. In this respect, the depth of modulation depends on the losses from the maximum achievable diffraction efficiency of the acousto-optical modulator.
In a different laser amplifying system of this type, the acousto-optical modulator is likewise operated in transmission in the resonator but the diffracted branch is used to couple out the laser power. In this embodiment, the frequency of the diffracted branch is shifted, on the one hand, and, on the other hand, the coupling out is dependent on the maximum achievable diffraction efficiency of the acousto-optical modulator. Furthermore, two diffracted beams are generally coupled out in the case of such resonators unless ring resonators are used, in which the radiation extends only in one direction.
Furthermore, systems of this type are known from Bonnet et al., Optics Communications 123 (1996), pages 790-800.
In the case of such laser amplifying systems, the diffracted branch of the acousto-optical modulator is coupled back in the resonator and the transmitted branch serves for the coupling out or contributes to the loss. In this embodiment, the frequency of the branch coupled back in the resonator is shifted and, on the other hand, the coupling back is dependent on the maximum achievable diffraction efficiency of the acousto-optical modulator.
The object underlying the invention is therefore to improve a laser amplifying system of the generic type in such a manner that this allows as efficient a division of the radiation field as possible without any complicated construction or complicated operation of the acousto-optical modulator.
This object is accomplished in accordance with the invention, in a laser amplifying system of the type described above, in that a modulation device is provided in accordance with any one of the embodiments described above, the radiation guide system of which forms the first amplifying radiation return system and the incident radiation field of which is the amplifying radiation field.
This solution likewise has the advantage that a modulation of the amplifying radiation field is possible with great efficiency with a simple construction of the acousto-optical modulator.
In this respect, it is, for example, conceivable for one of the exiting radiation fields of the modulation device to be coupled back into the optical amplifier.
When the inventive modulation device is provided for the coupling out of a radiation field this is preferably used such that the other one of the exiting radiation fields can be coupled out of the laser amplifying system.
Alternatively to the solutions of the inventive laser amplifying system described above or supplementary thereto, one particularly favorable solution provides for a first one of the amplifying radiation return systems to return to the acousto-optical modulator the branches which are incident in the first amplifying radiation return system and propagate in the acousto-optical modulator along their beam axes and which result in the acousto-optical modulator during the first acousto-optical modulation, that an angle between them corresponds approximately to the angle of diffraction of the first order, that the first amplifying radiation return system is arranged such that it returns to the acousto-optical modulator the transmitted branch formed in the acousto-optical modulator during the first acousto-optical modulation and the corresponding diffracted branch such that the returned transmitted branch forms approximately the same angle with the sound propagation direction as the diffracted branch formed during the first acousto-optical modulation and that the returned diffracted branch forms approximately the same angle with the sound propagation direction as the transmitted branch formed during the first acousto-optical modulation and that the returned transmitted branch and the returned diffracted branch extend in the acousto-optical modulator such that the transmitted and diffracted branches respectively resulting from the returned transmitted branch and the returned diffracted branch are superimposed to form a first radiation field and to form a second radiation field.
The advantage of the inventive solution is to be seen in the fact that the incident radiation field is divided into two radiation fields exiting from the acousto-optical modulator as a result of the inventive design of the first amplifying radiation return system in that that designated as first radiation field has the branches which have resulted on their way from the incident radiation field to the first radiation field as a result of a one-time diffraction and one-time transmission whereas that designated as second radiation field has the branches which have come about on their way from the incident radiation field to the second radiation field either as a result of no diffraction whatsoever or as a result of a two-time diffraction, wherein the first radiation field forms approximately the same angle with the direction of propagation of the sound waves in the acousto-optical modulator as the incident radiation field and the second radiation field forms with the direction of propagation of the sound waves approximately an angle altered in relation to the incident radiation field by the angle of diffraction of the first order so that, for example, with the inventive design of the first amplifying radiation return system different alignments of the first radiation field and of the second radiation field relative to the incident radiation field can also be achieved.
With the inventive solution, the acousto-optical modulator can preferably be operated together with the first amplifying radiation return system as a type of xe2x80x9canti-resonant ring interferometerxe2x80x9d, wherein a more or less complete coupling into the first, returning radiation field is already possible at a diffraction efficiency of the acousto-optical modulator of approximately 50% since the branches which have been diffracted one time and transmitted one time can be superim-posed constructively whereas the branch transmitted two times and the branch diffracted two times can be superimposed de-structively to form the second radiation field and thus cancel one another out, whereby an acousto-optical modulator can be used which is of a simple construction and operated with simple means.
The fact that an acousto-optical modulator with a low diffraction efficiency can be used efficiently makes a greater freedom in material selection and design possible.
The operation of the acousto-optical modulator together with the first amplifying radiation return system as xe2x80x9ca type of anti-resonant ring interferometerxe2x80x9d is to be understood such that the starting point for considerations is represented by the known xe2x80x9canti-resonant ring interferometerxe2x80x9d or also Sagnac interferometer, in which the acousto-optical modulator is used as a beam splitter, but it has to be taken into consideration, in addition, that the beam splitter is provided by a volume grating moving along in time and thus no defined, beam-splitting surface is present and an altered grating results, in addition, for the returned branches as a result of the transit time. Furthermore, the shift in frequency in the diffracted branch and the length of the path of the returned branches have also to be considered.
If the acousto-optical modulator is not acted upon with a sound wave and thus has the diffraction efficiency zero, the acousto-optical modulator no longer acts with the amplifying radiation return system as an anti-resonant ring interferometer on account of the transmitted branches exclusively forming and an essentially complete coupling of the incident radiation field into the second, divided radiation field takes place.
The inventive solution thus creates the possibility, despite an acousto-optical modulator which is of a simple construction and must only reach a diffraction efficiency of approximately 0% to 50%, of changing between maximum coupling to the first radiation field or to the second radiation field.
In principle, it would be conceivable to operate the acousto-optical modulator with two different diffraction efficiencies in order to achieve the desired modulation effects. For example, it would be conceivable to operate the acousto-optical modulator at a diffraction efficiency of approximately 0% and approximately 50%.
It is, however, particularly favorable when the acousto-optical modulator can be operated in the range of a diffraction efficiency of approximately 0% and approximately 50% so that all the possible, different degrees of coupling of the incident radiation field to the first and the second radiation fields can be set.
With the inventive solution, the acousto-optical modulator can form together with the first amplifying radiation return system an interferometer, with which, in contrast to the example designated as a type of anti-resonant ring interferometer, the two returned branches do not return to the modulator again in the opposite direction more or less along the identical path from the modulator via the first amplifying radiation return system but rather extend in such a manner that the first radiation field may be separated spatially from the incident radiation field.
With respect to the possibilities for the advantageous coupling of the incident radiation field to the first and the second radiation fields, for which acousto-optical modulators with a diffraction efficiency of approximately 0% to approximately 50% are adequate, the same statements apply as in the case of the construction as a type of anti-resonant ring interferometer.
A look at the symmetry of such arrangements shows that a further incident radiation field may be coupled in in an opposite direction in relation to one of the exiting radiation fields looked at previously and this radiation field is again divided into two exiting radiation fields, in principle, in the same way, one of these exiting radiation fields extending in a more or less opposite direction to the incident radiation field looked at previously.
With an arrangement in accordance with a type of anti-resonant ring interferometer two incident radiation fields can therefore be coupled in and these fields may be respectively divided into a first radiation field and a second radiation field, wherein the first radiation field extends each time more or less in an opposite direction to the corresponding incident radiation field whereas the second radiation field extends each time in a more or less opposite direction to the respectively other incident radiation field so that the first radiation field of the one incident radiation field exits each time in more or less the same direction as the second radiation field of the other incident radiation field.
For the arrangement deviating from the type anti-resonant ring interferometer, with which the exiting first radiation field is to be separated spatially from the corresponding incident radiation field, two respective pairs of incident radiation fields are accordingly conceivable, for which it is possible for the first radiation field of the one incident radiation field to extend in approximately the same direction as the second radiation field of the other incident beam of the same pair and in a more or less opposite direction each time to one of the incident radiation fields of the other pair.
With respect to how the feedback amplifier is intended to be operated, no further details have been given in conjunction with the preceding explanations concerning the individual advantageous embodiments of the inventive solution.
One advantageous embodiment, for example, provides for the first radiation field to be returned to the laser-active medium, i.e. the first radiation field with its frequency shifted is returned to the laser-active medium and thus no feedback of the radiation field resulting in the acousto-optical modulator in the form of an identical radiation field takes place but rather a feedback in the form of a radiation field with its frequency shifted, whereby properties differing from a known optical standing wave resonator can be formed.
In principle, it would be conceivable for the first radiation field to be coupled back into the laser-active medium via separate feedback elements.
The feedback of the first radiation field may, however, be brought about particularly favorably when the first radiation field is returned to the laser-active medium by means of the elements guiding the incident radiation field to the acousto-optical modulator.
As a result, a return of the first radiation field can be brought about in an advantageous manner without additional optical elements.
This is preferably possible when the first radiation field results within the acousto-optical modulator in an area which is located within the optically active volume area so that the first radiation field results such that locally it coincides essentially with the incident radiation field but propagates in an opposite direction to it.
Another advantageous solution consists in the second amplifying radiation return system being designed such that the first radiation field is returned to the incident radiation field again through it.
Such a feedback of the first radiation field to the laser-active medium is possible, in particular, when the first radiation field has a power greater than zero on account of diffraction effects in the case of an acousto-optical modulator operating at a finite diffraction efficiency.
A particularly high feedback by means of the first radiation field results when the acousto-optical modulator operates with a diffraction efficiency of approximately 50%.
A further, advantageous solution in the case of the inventive laser amplifying system provides for the second radiation field to be returned to the laser-active medium. Such a return has the advantage that the frequency of the second radiation field is not shifted and thus this creates the possibility of bringing about a feedback with a radiation field of identical frequency.
Such a feedback may take place in the most varied of ways. It is particularly favorable when the second radiation field is returned to the laser-active medium by being returned to the acousto-optical modulator and via this as well as the first amplifying radiation return system; as a result a return can be brought about in a particularly favorable manner without many additional components.
It is particularly favorable when the second radiation field is returned to the laser-active medium due to reflection into itself so that the same optical components which contribute to the formation of the second radiation field can essentially be used.
Such a return of the second radiation field always takes place when the resulting second radiation field does not have the power zero.
A feedback to the laser-active medium which is, in particular, essentially complete occurs when the total power of the incident radiation field is essentially found again in the second radiation field so that the essentially entire power enters the laser-active medium due to reflection of this second radiation field back into itself or due to a return of this second radiation field via the active medium to the first radiation field again.
This is the case, in particular, when the acousto-optical modulator operates with a diffraction efficiency of approximately zero or approximately 100%.
In order, in particular, to be able to couple out laser radiation favorably, a particularly advantageous embodiment of the inventive laser amplifying system provides for the first or the second radiation field to form the radiation field returned to the laser active medium and for the respectively other radiation field to form the radiation field coupled out.
With this solution it is possible to use one of the two radiation fields as a radiation field coupled out and the other for the feedback to the laser-active medium.
Which of the two radiation field is, in the end, coupled out or not depends on the individual possibilities for realizing the inventive laser amplifying system.
In the case of several realization possibilities it is advantageous to feed the first radiation field and thus the radiation field with its frequency shifted back again into the laser-active medium whereas in the case of other embodiments, in particular, when an optimum feedback is desired, to couple out the second radiation field, the frequency of which is shifted, so that a frequency-identical feedback to the laser-active medium can be realized.
With respect to the manner, in which the first amplifying radiation return system acts on the branches forming in the acousto-optical modulator, different solutions are conceivable.
With respect to the position of the returned branches in the acousto-optical modulator, the most varied of solutions are conceivable.
One possibility provides for the returned branches to be aligned in the acousto-optical modulator such that one of the resulting radiation fields, preferably the first radiation field, can be coupled into the laser-active medium.
The first radiation field preferably propagates in the direction of the laser-active medium essentially overlapping with the incident radiation field so that it can be guided through the same optical elements as the incident radiation field.
With this solution the second radiation field can be used directly as a radiation field which is coupled out when the first radiation field is fed back into the laser-active medium.
If, on the other hand, the second radiation field is to be coupled back into the laser-active medium, the first radiation field will, for example, be separated from the incident radiation field as a result of polarization effects, in particular, splitting of the radiation field into two polarization components oriented orthogonally to one another as well as polarization rotations or phase shifts between the polarization components.
Another possibility is to align the returned branches in the acousto-optical modulator such that the first radiation field is to be separated spatially from the incident radiation field in that at least an angular offset or a space exists between the two radiation fields.
With this solution, the first radiation field is preferably used as a radiation field coupled out whereas the second radiation field can be coupled into the incident radiation field due to reflection back.
Another advantageous solution provides for the second amplifying radiation return system to be designed such that the second radiation field is guided back through it again into the incident radiation field.
One advantageous embodiment provides for the first amplifying radiation return system to align the returned branches such that they run towards one another in the acousto-optical modulator so that the transmitted and diffracted branches resulting from them again result lying as close as possible next to one another.
This is preferably brought about such that the returned branches intersect one another again in the optically active volume area within the acousto-optical modulator at least in sections, even better essentially completely.
Such a radiation guide system can be designed in the most varied of ways.
The returning of the individual branches would, for example, be possible due to the fact that the transmitted branch is coupled into one end of an optical waveguide and the diffracted branch into the other end and the respective returned branches then exit again at the opposite ends.
Another solution provides for the first amplifying radiation return system to have at least two beam deflections which cause the branches running apart from one another at the angle of diffraction to run towards one another again at the angle of diffraction as returned branches.
The beam deflections are preferably designed as reflector surfaces.
In this respect, it is preferably provided for the two reflector surfaces to form with one another an angle of less than 90xc2x0.
Another solution provides for a reflector and an optical imaging device, for example, comprising two lenses.
In the simplest case, it is sufficient when the respective returned branch extends in the acousto-optical modulator parallel to the respectively other branch.
A particularly favorable solution, in particular, with a view to the formation of a returning radiation field which is as uniform as possible from a spatial point of view and a radiation field coupled out which is as uniform as possible from a spatial point of view provides for the first amplifying radiation return system to cause the returned branches to extend in the optically active volume of the acousto-optical modulator such that they intersect at least partially approximately in the area of a division into the transmitted and the diffracted branches, wherein the one respective returned branch preferably extends in the optically active volume area within the acousto-optical modulator approximately congruent but with an opposite direction of propagation to the respectively other branch. As a result, the transmitted and diffracted branches again resulting from the returned branches also coincide essentially with one another.
Another alternative solution provides for the amplifying radiation return system to return the returned branches as branches running apart from one another in the acousto-optical modulator, i.e. the respective returned branches, formed, for example, due to reflection of the transmitted branch as well as the diffracted branch, extend in the optically active volume area within the acousto-optical modulator as branches running apart from one another.
It is also conceivable with this relative orientation of the returned branches to couple the transmitted branch and the diffracted branch into a respective light guide and have them exit from it again at the other end with a corresponding alignment.
A particularly simple amplifying radiation return system is preferably constructed such that it has a single reflector.
The one reflector is preferably aligned such that a radiation field impinging on it is reflected back at an angle of return reflection which corresponds to the angle of diffraction of the first order of the acousto-optical modulator.
In this respect, the reflector is preferably designed such that it has a flat reflector surface which is aligned in accordance with the angle of return reflection.
The reflector surface can be part of a reflector separate from the acousto-optical modulator.
Another advantageous solution provides for the acousto-optical modulator to bear the reflector on a side surface on the exit side for the radiation field incident in it.
In the simplest case, such a reflector may be produced on the side surface of the acousto-optical modulator on the exit side when the side surface of the acousto-optical modulator is covered, preferably by way of vapor deposition, with a reflector layer.
In all the variations of the inventive solution, with which the returned branches likewise enter the optically active volume area of the acousto-optical modulator as branches running apart from one another, the branches forming not only the returning radiation field but also the radiation field coupled out are offset in a direction transverse to their direction of propagation.
This offsetting is, however, preferably within the cross section of the radiation field forming altogether so that this does not have any appreciable effect when the reflector layer is arranged close to a junction of the branches in the acousto-optical modulator, i.e. the acousto-optical modulator has an extension in the direction of propagation of the incident radiation field which is as limited as possible and necessary only for an adequate interaction.
The inventive laser amplifying system in accordance with all the embodiments described thus far can be operated with the most varied of operational modes.
It is, for example, conceivable to use the laser amplifying system as a laser radiation source, wherein the acousto-optical modulator serves to vary the quality within the resonator.
In this case, it would, for example, also be conceivable to realize the coupling out of the usable laser beam in any known way. For example, it is also possible to realize the coupling out by means of a second acousto-optical modulator which could, for example, also interact in an inventive way with the second amplifying radiation return system.
Another advantageous embodiment provides for one of the radiation fields exiting from the acousto-optical modulator to supply the usable laser radiation, wherein either a partial coupling out of the laser radiation building up in the resonator or a complete coupling out can take place.
Another advantageous embodiment of an inventive laser amplifying system is likewise used as a laser radiation source, wherein, in this case, the acousto-optical modulator is used to vary the degree of coupling out in different ways.
One embodiment utilizing this effect advantageously provides for the pulse energy which would be contained in a normal Q-switched pulse to be divided amongst a group of several pulses, i.e. the laser amplifying system to be operated in a so-called burst mode.
Finally, a further possibility of using the inventive laser amplifying system provides for this to likewise be operated as a laser beam source with a mode coupling, wherein it is possible to utilize the fact that the frequency of the returning radiation field is shifted in comparison to the incident radiation field.
Furthermore, the laser amplifying system may be advantageously used for amplifying a radiation field coupled in from outside, such as during injection seeding or during regenerative amplification. The fact that the frequency of the returning radiation field is shifted in comparison with the incident field may also be utilized in the case of such an amplification in multiple transit.